1. Preheat oven to 180C, Gas 4.
2. Prepare a small rectangular roasting tin or oven proof dish approximately 8 x 12″ (20 x 30cm). Grease or line with baking parchment.
3. Melt the butter and chocolate over a pan of simmering water. Cool slightly.
4. Stir in the sugar and vanilla.
5. Add the eggs, one at a time, stirring until well blended each time.
6. Sift the flour and cocoa together, add to the mixture and beat for about 1 minute, until smooth.
7. Pour into prepared tin, and bake for about 45 – 50 minutes. The mixture needs to be just cooked, so start testing with a cocktail stick at about 40 minutes – it should have moist crumbs, but not wet batter, still clinging to it.
8. Cool in tin until cool enough to handle, then cut into pieces, but leave until completely cold before removing from tin.